In the right applications, powder injection molding (PIM) can turn out metal parts that perform better and cost less than those produced through traditional metal manufacturing processes. But what exactly makes a part a good candidate for PIM and the benefits it can bring?
Randall German, a PIM expert and director of the Center for Innovative Sintered Products at Pennsylvania State University, recently provided some easy answers to this tough question. Speaking at the Metal Powder Industries Federation's annual conference in Las Vegas, German outlined some of the key factors design engineers should consider when evaluating designs for PIM.
These "simplified design rules" aren't intended to replace the kind of advanced design and manufacturing know-how required as a product nears production. Instead, German's remarks focused on assisting design engineers with the upfront "identification of the features, materials, sizes, and shapes that best fit with powder injection molding technology."
He managed to boil this initial decision-making process down to 10 design factors that favor PIM:
1. Low effective density. Effective density, or the mass of a component divided by volume of the envelope from which machining would start, serves as an indicator of wasted material and machining time-and cost. According to German, successful PIM tend to have an effective density of 25% or less. "PIM is excellent at reducing fabrication costs for components with low effective density," he says.
2. Adequate production volumes."Tooling and set-up costs are not justified for low production quantities," German says. You should start to look at PIM only when annual production quantities are at least 20,000 parts per year, though some successful jobs have had volumes as low as 5,000 when other factors favored PIM.
3. The right material. PIM gets more attractive when designs require hard-to-machine materials. German says this is especially true for metals with multiphase microstructures or those that exhibit high work-hardening rates. "Try to avoid strong oxide formers, reactive metals, and volatile and toxic metals," German advises, citing beryllium, lead, manganese, and magnesium as a few poor candidates. Also, he adds that PIM works best with materials that melt at temperatures higher than 1,000C, though aluminum is an exception in thermal management applications.
As for the materials that PIM supports, there are now many-including ferrous alloys, common oxide ceramics, tungsten alloys, and various cermets. "Besides these materials, there is emerging activity in special materials," German says, citing aluminum, precious metals, titanium alloys, nickel-based super alloys as just a few examples.
4. Geometric complexity and size.German describes PIM as most fitting for components with moderately complex geometries. "It works best on components that require multiple axes for indexing if machined," he explains. Looked at differently, PIM targets parts with between 10 and 100 engineering specifications-whether they relate to dimensions or other call-outs on the print. "The most challenging components currently in production have 130 dimensional specifications," German notes.
Size-wise, PIM parts still tend to be on the small side. In a summary of 220 commercial parts, German found that the median component is just over one-inch maximum dimension with a weight of eight grams. "PIM excels at forming smaller components where much mass removal would be required by machining," he says.
5. Mechanical performance. German reports that the strength properties of sintered PIM parts typically match or exceed those obtained from other manufacturing routes. "Simply put, PIM delivers essentially handbook properties," he says, adding that this rule has some exceptions for those materials that attain their strength through deformation processes, like forging. PIM products also exhibit machineability, thermal behavior, wear resistance, and corrosion resistance comparable to parts made with conventional metal technologies.
6. Surface finish requirements. Initial particle size largely governs the surface roughness of PIM parts. "Accordingly, controlled textures are possible at almost no cost penalty," German notes.
7. Tolerance troubles. If traditional production methods have a tough time holding tolerances on certain features, PIM often becomes a stronger candidate. "PIM is best suited to situations where certain features will cause difficulties or low process yields when using alternative production techniques," German says.
8. High assembly costs. PIM offers parts consolidation opportunities, potentially averting costly assembly operations and inventory costs.
9. Blemish tolerance.PIM can produce some production blemishes, near gates, ejector pins and parting lines. German says to make sure you position such defects in non-functional areas of the part or be ready to remove them in secondary operations.
10. Material combinations.By mixing powders, PIM can tackle novel material combinations that are difficult via traditional metal working processes, German adds. He gives laminated structures and mixed metal-ceramic materials as two examples.
More detailed design information can be found in German's paper "Design Guide for Powder Injection Molding-Simplified Rules," available from the MPIF at http://www.mpif.org.
Or visit CISP at http://www.cisp.psu.edu/